High technology industries have developed numerous uses for precisely controlled electrical motors. One of the most popular types of these motors is the brushless magnetic motor which is frequently used in computer systems and peripherals. Due to the high precision required in these applications, it is desirable to obtain extremely accurate measurements as to the rotational velocity of the motor at all times. The velocity measurement provides an analog for position sensing and thus permits servo control of the mechanism to which the motor is attached.
Various devices have been utilized in the prior art to sense the velocity of rotating motors. Optical methods of scanning a rotating portion of the motor, and thereby sensing velocity, have been disclosed in U.S. Pat. No. 4,228,387, issued to W. S. Brown and also in U.S. Pat. No. 4,258,622, issued to S. Estrabaud, et al. Optical methods such as these require exterior sensing devices and relatively complex decoding components. This type of device often significantly adds to the cost and complexity of the motor assembly.
Another common velocity sensor method utilizes the Hall effect to provide analog velocity measurements. The Hall effect devices incorporate the response of certain semiconductor materials to magnetic field variances. The prior art Hall effect devices have limited frequency response and are expensive and difficult to manufacture for high accuracy applications. Hall effect sensors are also single point sensing devices which do not cancel out minor concentricity errors in the magnet.
Magnetic tachometer devices have also been frequently used in the prior art for sensing rotational velocity. One example of a prior art magnetic tachometric device is disclosed in U.S. Pat. No. 3,504,208, issued to J. D. Rivers. Prior art magnetic tachometer devices such as those disclosed by Rivers require complex assembly, and require commutating brushes which tend to increase system friction and require periodic replacement. In this manner, the prior art magnetic tachometric devices have limited usable lifetimes. Furthermore, they are not desirable in high velocity constant use applications, such as data processing applications of high speed electrical motors.
One further method of magnetically sensing the rotational velocity of a motor utilizes a plurality of magnetic poles rotating with the meter in relation to a series of induction coils or traces. The rotation of the poles with relation to the conducting elements induces a current within the conducting elements which may be measured as an analog for the velocity of the motor. One application of this type of rotational velocity sensor is disclosed in the co-pending and co-assigned applicatioin of Kenneth Kordik for an "Improved Magnetic Rotational Velocity Sensor." Such a sensor is particularly for use with a stepper motor and not with continuous velocity rotational motors.
Applicant is further aware of the use of a separate ring magnetically polarized to include a plurality of alternating magnetic poles, which is attached to the motor in such a manner that it rotates past a circuit board including a number of conducting traces in which the current is induced. Such applications have been utilized in those electrical motors wherein the main motor magnet utilizes an axial field. In this manner, the tachometric alternately polarized ring also has an axial field. In these applications the main motor magnet and the alternately polarized tachometric ring are separately manufactured items.
The prior art rotational velocity sensors commonly share the disadvantages of complexity and difficulty of manufacture. Those devices which are simple and easily manufactured do not have sufficient resolution to accurately measure the rotational velocity of high speed electrical motors. Attempts to increase the number of motor magent poles to achieve higher frequency measurement leads to significant increases in cost. None of the prior art methods solve all the problems relating to extreme accuracy, low cost and ease of manufacture.